Concentric line tuned circuits



Dec. 5, 1939.

70 CA Tl/UD 0F TUBE J. W. CONKLIN CONCENTRIC LINE TUNED CIRCUITS FiledJan. 50. 1937 ALTER/WW5 r0 GRID DIAHT Q or VAtl/l/M com/5mm r055INDUCTIVE {i 0I/A/E'CT/0Il .1

r0 (4 mom INVENTOR JAMES W. CONKLIN VBY) [M ATTORNEY Patented Dec. 5,1939 UNITED STATES CONCENTRIC LINE TUNED CIRCUITS James W. Conklin,Audubon, N. J., assignor to Radio Corporation of America, a corporationof Delaware Application January 30, 1937, Serial No. 123,096

19 Claims.

The present invention relates to tuned high frequency oscillatorycircuits, and more particularly to such of these as are known by theterm concentric conductor. resonator or line resonator. By the termconcentric conductor resonator is meant a tuned circuit having inner andouter, substantially concentric conductors so coupled together as toform an oscillatory circuit. Examples of these are found in ReissuePatent No. 20,189, granted December 1, 1936, to Hans Otto Roosenstein,and in the article Resonant Lines for Frequency Control by Clarence W.Hansell, published in the A. I. E. E., August, 1935, pages 852 to 857,to which reference is herein made for a more detailed descriptionthereof.

In my United States Patent No. 2,104,554 and 2,124,029, granted January4, 1938, and July 19, 1938, respectively, and in the above mentioned A.I. E. E. article, are described two types of concentric resonatorshaving inner conductors of double diameter, the resonators beingdesigned to reduce the overall length of the tuned circuit considerablybelow that of a full quarter-wave counterpart. Essentially theseresonators have an inner conductor of two diameters, i. e., a smalldiameter section which constitutes an effective inductance of minimumlength necessary to attain the desired input impedance, and a largediameter section of substantially equal length as said small diametersection, but of a diameter such as to provide a proper effectivecapacity for resonating at the desired frequency. The entire circuit ofinner and outer conductors resonate in the manner of a parallelresonance circuit, in the sense that the inductive reactance of theinductive section and the capacitive reactance of the capacity sectionwill be formed wholly or in part by the correlative functions of theinner and outer conductors or component parts thereof. These concentricresonators having inner conductors of double diameter, as described inthe above mentioned patents, are provided with means for maintaining theoverall length of inner conductor constant, whereby there is obtainedsubstantial freedom from the effects of variations in .the ambienttemperature on the resonant frequency. Although such tuned concentricresonators have proved to be satisfactory, and not objectionably largefor use with transmitting equipment, they are still inconveniently largefor use with receiving or low power equipment.

The present invention overcomes the foregoing disadvantage and has forits primary object to provide a concentric conductor resonator which isextremely small and compact, and has a mini-- mum overall lengthcommensurate with its associated features.

In my above mentioned United States patents, and in my United StatesPatent No. 2,103,515, granted December 28, 1937, are shown Variousmethods of obtaining a fine adjustment of the resonant frequency of theconcentric conductor resonators. These are satisfactory for transmitterfrequency adjustment, where a sensitivity of adjustment of one part inten thousand, or alternatively, several hundred cycles is sufiicient.(The expression sensivity of adjustment should not be confused withfrequency stability since the former expression refers to the ease withwhich the system can be adjusted mechanically to a desired frequency,whereas the latter expression concerns the constancy of that frequencyonce it is set.) However, such methods are not suitable where a fineradjustment of sensitivity is desired, for example, where we wish anadjustment sensitivity of less than one part in five million, as isrequired in the case of controlling the frequency of two beating highfrequency oscillators within several cycles when producing an audiofrequency beat.

Accordingly, another object of my invention is to provide a concentricconductor resonator having adjustment means of greater sensitivity andsmoothness than heretofore employed.

A further object is to provide adjusting means for a concentricconductor resonator whereby the resulting change in the resonantfrequency produced by said adjusting means shall have an approximatelylogarithmic relation to the movement of the adjusting control.

A still further object is to provide a reduced size of doublediameter-inner conductor concentric resonator by greatly increasing thecapacity between the larger diameter section of the inner conductor. andthe outer conductor.

In general, the invention comprises a concentric conductor resonatorconstructed with two sections of different diameter for the innerconductor, the larger diameter section being conductively connected tobut folded back upon the smaller diameter section in such manner thatthe overall length of line required to tune to a given frequency isgreatly reduced. In order to still further reduce the overall length ofline of the resonator, there is provided, in accordance with oneparticular embodiment of the invention, an additional metallic cylinderlocated within the large diameter section of the inner conductor andconductively connected to the outer conductor of the resonator throughthe base or end plate thereof for increasing the capacity between thelarge diameter section of the inner conductor and the outer conductor.

A feature of the invention lies in the arrange ment of the electricallyconductive flexible bellows connection which connects with the twodifferent diameter sections of the inner conductor of one embodiment ofthe invention.

A further feature of the invention resides in the anti-friction thrustbearing mechanism for obtaining micrometer adjustments, together with acoil spring arrangement for eliminating back lash.

Other features reside in the method of and means for obtaining absolutetemperature compensation of the resonator at the resonant frequency.

Still further objects and features together with various advantages willappear from a reading of the following description, which is accompaniedby a drawing wherein:

Fig. 1 illustrates a cross sectional view of a concentric conductorresonator in accordance with the invention, and

Fig. 2 illustrates a modification thereof.

Referring to Fig. 1 in more detail, there is shown a tuned oscillatorycircuit comprising a concentric conductor line resonator having an outerconductor I constituting, in addition, the capacity member as well asthe support and shielding for the entire assembly, and an innerconductor having a small diameter section 2 and a larger diametersection 3 folded back over the smaller diameter section. These twosections of different diameters of the inner conductor need not be ofthe same length but in the practical case will usually be approximatelythe same length to ffect maximum utilization of space. In general, asection of short circuited transmission line presents an inductive inputimpedance when less than one-quarter wave long. As the length approachesone-quarter wave, the impedance increases and shifts phase first to ahigh resistance and then to equivalent capacity reactance as the lengthincreases beyond a quarter wave to a half wave, where the reactancepasses through a minimum value and changes to inductive again andrepeats the process. In practice, the smaller diameter section 2 of theinner conductor (i. e., inductive section), is preferably less thanone-quarter wavelength although it might be desirable in some cases touse a longer conductor, an odd multiple of a quarter wave long. Thelength would have to be between zero and one-quarter wavelength, betweena half and three-quarters wavelength, between one and onequarterwavelengths, etc., and not between multiples of some shorter length. Thelarger diameter section 3 of the inner conductor is the capacitivesection, both sections of inner conductor resonating at the desiredfrequency in the manner of a parallel resonance circuit. Inner conductor2, 3 is conductively coupled to the outer conductor I by means ofmetallic end plate I9, which supports the inner conductor and itsassociated adjusting equipment. At the opposite end of outer conductor Iand fixed thereto by means of screws 2!], 2II, is the end plate II whichforms one member of an adjusting capacity comprising plate II and plateI3, and which end plate is removable for convenience in assembling theresonator.

The small diameter section 2 of the inner conductor, which is mountedrigidly to the outer shell I by nut 8, is made of a proper length todevelop the desired terminal or input impedance to the resonator. Tuningof the resonator is effected in part by the capacity existing betweenthe cylinder 3 and the outer conductor I. This cylinder 3 isinterchangeable for accomplishing major changes in frequency and isaflixed to the inner conductor section 2 by nut I5. A damping ring 4 ofsuitable insulating material furnishes further lateral support tocylinder 3 and prevents mechanical vibrations due to a bell action ofthis cylinder. Most of the remaining tuning capacity is formed by themoving plate I3 and the end plate I I. An extensible electricallyconducting bellows I4 forms a flexible connection between plate I3 andthe main capacity outer cylinder 3. There is some stray capacity betweenthe bellows and the outer cylinder but this is incidental to theoperation of the resonator. The primary connection of the capacitycylinder 3 (Fig. 1) to the small diameter inner conductor 2 is throughthe metallic plate 24 which is adjacent ring I5. An alternative way ofvisualizing this type of resonator is to consider this plate 24 as thebase of a section of line of which cylinder 2 is the inner conductor andcylinder 3 the outer conductor, and inductive reactance therebyconstituted tuned to the desired frequency by the capacity formedbetween cylinders I and 3 plus the capacity between plates II and I3 andthe bellows capacity. The range of movement and size of the end plate I3and bellows I4 is determined by the proportionate capacity required tobe variable to cover the desired frequency range and the sensitivity ofadjustment, i. e., a given capacity change could be accomplished eitherwith a small spacing and small movement or greater spacing and greatermovement. Where it is desired to cover approximately a 5% frequencyrange, the required sensitivity of adjustment necessitates a large rangeof movement. This necessitates having approximately 10% of the totalcapacity variable and a large diameter plate to obtain this capacitywith the desired range of movement.

In the adjusting mechanism, a fine pitch micrometer screw It operatesthrough an antifriction thrust bearing I having balls I8 bearing againstflange It on thrust rod 5 compressing coil spring 5 and moving capacityplate I3. The coil spring removes all backlash from the mechanism. Inprevious types of adjusting mechanism, without the anti-frictionbearing, it was found that a coil spring, to remove backlash, causedhigh starting friction on the adjusting screw making the action jerky,and a push-pull type of operating screw with unavoidable backlash of thetype disclosed in the above mentioned United States patents, was used asthe best compromise. Thrust rod 5 also serves to rigidly maintain thealignment of the capacity plate I3 without interfering with freedom oflongitudinal movement by virtue of the widely separated bearings at ITand It between which bearings the rod 5 extends as an integral unit. Itwill also be observed that the hole AI in end plate I3 whichaccommodates screw I2 is somewhat larger than the screw itself in orderto permit lateral alignment in assembling and avoiding lateral pressureon the bearings as far as possible.

It is well known that the capacity between two plates, such as I I andI3, depends in one degree on the spacing between the plates and ingeneral is inversely proportional to the spacing. Thus, a given changein the spacing between the plates will have a large change in thecapacity when the total spacing is small and a small change in thecapacity when the total spacing is comparatively large. By properselection of the limits and range of movement, it is possible toapproach very closely to a logarithmic change of capacity over aselected range. For small capacity changes, the percentage change inresonant frequency is approximately equal to half the percentage changein capacity. Therefore, it is possible by proper design, and by makingthe total spacing small compared with the total adjustment movement, toeffect an approximately logarithmic rate of change of resonant frequencywith movement of the adjusting screw. This characteristic is verydesirable in a beat frequency oscillator in order to spread out the lowbeat frequency range. In case such a characteristic is not desired, bymaking the total spacing large compared with the total adjustmentmovement, an approximately linear rate of change of frequency can beobtained with the adjusting screw.

Two types of temperature compensation can be attained with this type ofresonator. It may be compensated in such a way that the difference inresonant frequencies of two similar resonators of the invention isunaffected by changes in ambient temperature, or it may be compensatedto have the resonant frequency of a single resonator unaffected bychanges in ambient temperature at a particular frequency. For beatfrequency oscillator application, within limits, the absolute frequencyis not important but it is very desirable that the beat frequency shouldbe independent of ambient temperature.

Where two identical resonators of the type of the invention are to beused for independently controlling the frequency of two beatingoscillators in a beat frequency oscillator, it is assumed that the tworesonators are subjected to the same ambient temperature and the zerobeat adjustment will be with the adjusting capacity plates insubstantially identical positions. Therefore, for zero beat the twooscillators will have identical temperature characteristics and willhold zero beat with varying ambient temperature. Presuming that the zerobeat position is the minimum capacity (maximum spacing) adjustment, itcan be shown that the beat frequency will be unaffected by changes inambient temperature for a particular beat frequency by maintaining thespacing of the capacity plates constant against change by temperature.If the whole resonator were constructed of the same material, thespacing would have the same coefiicient of change as the material. Tocompensate for this, it is only necessary to select materials orcombinations of material for the adjusting screw and thrust rod to havethe same overall net linear expansion with changes in temperature as theouter shell of the resonator for a particular spacing. It is thuspossible to compensate a beat frequency oscillator for the extremeranges of the beat frequency and have it fairly well compensated overthe entire range.

When a single resonator of this type is to be used to rigidly stabilizeor control a single oscillator at a given definite frequency, absolutetemperature compensation of the resonant frequency is required. It maybe assumed that a large capacity spacing will be used compared with thetotal range of adjustment. It may be shown that under these conditionsthe resonant frequency will be unaffected by variations in ambienttemperature when the coefficient of expansion of the thrust rod andadjusting mechanism is made equal to:

so w) where a is the temperature coefficient of the material forming theinner and outer conductors,

S is the spacing of the adjusting capacity betwen plates II and I3,

L is the total length of the inner conductor assembly,

C is the total capacity of the inner conductor assembly,

and

C is the capacity of the adjusting plates II and Since this coefiicientis less than that of the material of the conductors, it is comparativelyeasy to obtain by using a composite thrust rod, apportioning its lengthwith materials having two different coefficients to obtain the desiredcoefiicient. On the other hand, since C is proportional to the area ofthe plates divided by the spacing S, it is possible, by varying the areaor the spacing, or both, to obtain any desired coefficient. Thus, if itis desired to use any particular material for the thrust rod, it is onlynecessary to design the adjusting capacity so that the quantity is equalto the coefficient of expansion of that material. must always have alower coefficient than the conductor elements. However, inasmuch as theconductors will generally be made of copper, there are many desirablematerials available, such as steel, having a lower coefiicient.

Alternative connections to the inner conductor from electron dischargedevice equipment have been indicated by dotted lines in Fig. l, as theymay be made at any convenient point along the capacity cylinder 3through a suitable hole or insulating bushing in the outer conductor I.

In Fig. 2 there is illustrated a modification of the circuit of Fig. 1.This figure eliminates the use of the flexible bellows between the twodiameter sections 2, 3 of the inner conductor, and provides anadditional cylinder 2I within the larger diameter section 3 andconductively coupled to the outer conductor 1 through the base or endplate I9 for increasing the capacity between the large diameter section3 of the inner conductor and the outer conductor I, with a consequentreduction in the length of the concentric line resonator. The additionof the inner conductor 2| is equivalent, so far as capacity isconcerned, to increasing the length of the large diameter section 3, butsuch an arrangement, it is to be distinctly understood, does not changethe effectiveness of temperature compensation which is stillproportional to the length of the inner conductor. of the adjustingcapacity existing directly between the plate I3 and end plate II, as inFig. 1, there is now provided an adjusting metallic plate 22 mounted ona screw 23 for providing the adjusting capacity between plate I3 and endplate It will be observed that this material It will be noted thatinstead is a specific design for use where thermal compensation isconsidered unnecessary, and consequently the bellows arrangement of Fig.1 is omitted for simplicity. It will thus be evident that there is noneed for a thrust rod as in the case of Fig. 1, all adjustment in Fig. 2being effected by means of moving plate 22. v

The following advantages are obtained in the concentric conductorresonator of the present invention: (a) Since its diameter is determinedby the desired power factor (the considerations in this respect are thesame as for a quarter wave line of the same conductor diameters), itsoverall length is only slightly greater than the minimum required toobtain the desired input impedance on the inner conductor. (1)) As onlya small part of the total capacity is adjusted, and that with precisionsmoothness, high adjustment sensitivity is possible. (0) The adjustmentmay be made to have a substantially logarithmic adjustmentcharacteristic for such applications as a beat frequency oscillator.(03) It is possible to compensate for variations in the ambienttemperature either to hold constant resonant frequency or to hold aconstant difference frequency with a similar resonator, such as in abeat frequency oscillater or superheterodyne receiver. (e) Connection tothe inner conductor can be made through a very short lead at practicallyany point throughout the length of the resonator. (f) Its constructionand assembly are simple and conveniently adapted to standard materialsizes and simple machine operations. (9) Since the ca-- pacity areas arelar e, close spacings are unnecessary and it will be comparatively freefrom troubles due to mechanical shocks or vibrations.

The tuned concentric lines of l and 2 can be coupled to a vacuum tube inany desired manner by direct, inductive, or capacitive connec tions.Both figures show one way of directly coupling the folded back, largerdiameter section 3 of the inner conductor to the grid of a vacuum tube,when such a tuned concentric line is to be used as the frequencycontrolling element of an oscillator. It will be noted that these directconnections extend through a suitable hole in the outer conductor to thelarge diameter section of the inner conductor. Where, however, it isdesired that a lower impedance tap be used, the direct connection may bemade through holes in both the outer conductor and the large diameter orcapacity section of the inner conductor whereby connection may be madeto the small diameter section of the inner conductor. Such anarrangement is shown in dotted lines in Fig. 1. It will be understood,of course, that where desired other types of connection may be employed,such as an inductive connection in the manner also shown in dotted linesin Fig. 1, and that the tuned concentric line is not limited sole- 1yfor use as a frequency controlling element of an oscillator since it canbe employed as an impedance coupling element bet Jeen stages of a radiotransmitter or receiver. or as a substitute for a tuned oscillatorycircuit wherever such an oscillatory circuit can be use What is claimedis: i 1. An oscillatory circuit comprising a concentric line resonatorhaving an inner and an outer conductor, said inner conductor having twosections of different diameters, the larger diameter section beingfolded back over the smaller diameter section for approximately theentire length of said smaller diameter section, said sections beingconductively connected together at only one of their adjacent ends.

2. An oscillatory circuit comprising a concentric line resonator havinginner and outer conductors conductively coupled together at one end,

section being folded back over the 3. An oscillatory circuit comprisinga concentric line resonator having inner and outer conductors coupledtogether at one end, said inner conductor having two sections ofdifferent diameter, a flexible conductive bellows comprising anextension of the larger diameter section of the inner conductor toincrease the length thereof, the smaller diameter section being coaxialwith said outer conductor and supported at one of its ends by said outerconductor, the larger diameter section being folded back over thesmaller diameter section for approximately the entire length of saidsmaller diameter section, said sections being connected together at onlythe other end of said smaller diameter section.

4. An oscillatory circuit comprising a concentric line resonator havingan inner and an outer cylindrical conductor, means for conductivclycoupling said two conductors together at one end to constitute a tunedcircuit, said inner conductor of said tuned circuit having two sectionsoi different diameter conductively coupled together end to end, thelarger diameter section being folded back over the smaller diametersection, and a cylindrical conductor located between the two diametersections of inner conductor and coupled to said outer conductor at saidone end for increasing the capacity between said inner and outerconductors.

5. An oscillatory circuit comprising a concentric line resonator havingan inner and an outer tubular conductor rigidly connected together atone end, said inner conductor having two sections of different diametersconductively coupled together, the larger diameter section being foldedback over the smaller diameter section and held in position by ametallic plate extending substantially the width of the larger diametersection, adjusting mechanism for adjusting the effective length of saidinner conductor comprising a thrust rod within the smaller diametersection extending from said one end of the resonator to said metallicplate, a bearing at each end of said thrust rod, a compressed coilspring located between one of said bearings and a flange on said rod,and a line pitch micrometer screw engaging said other bearing.

6. An oscillatory circuit comprising a concentric line resonator havinganinner and an outer tubular conductor rigidly connected together at oneend, said inner conductor having two sections of different diameters,the larger section being folded back over the smaller diameter section,a first end plate having a width substantially the width of said largerdiameter section conduc tively coupling said two diameter sections togather at the end of said smaller diameter sec tion remote from said oneend, a second plate conductively connected to the other end of saidouter conductor and forming with said first end plate a capacity,adjusting mechanism at said rigidly connected end including a thrust rodlocated within said smaller diameter section extending from said rigidlyconnected end to said first end plate, the coefiicient of expansion ofsaid thrust rod and adjusting mechanism being equal to H a LC Where a isthe temperature coefficient of the material forming the inner and outerconductors, S is the spacing between said two plates, L is the totallength of the inner and outer conductor assembly, C is the totalcapacity between the inner conductor assembly and the outer conductor,and C is the capacity of the two plates.

7. A tuned circuit comprising inner and outer conductors, said innerconductor having two sections, an inductive and a capacitive section,the whole tuned circuit resonating at the desired frequency in themanner of a parallel resonance circuit, said capacitive sectionextending over substantially the entire length of said inductivesection.

8. A tuned circuit comprising inner and outer conductors, said innerconductor having two sections, an inductive and a capacity section, saidinductive section consisting of a relatively short conductor less thanone-quarter wavelength in length, and said capacitive section consistingof a conducting surface in close proximity with the outer conductor andforming a capacity therewith, the Whole tuned circuit resonating at thedesired frequency in the manner of a parallel resonance circuit, saidcapacitive section extending over substantially the entire length ofsaid inductive section.

9. A tuned circuit comprising inner and outer conductors, said innerconductor having two sections, an inductive and a capacity section, saidinductive section consisting of a relatively short conductor less thanone quarter wavelength in length, and said capacitive section consistingof a conducting surface directly connected at one end to said inductivesection and surrounding the same for substantially its entire effectivelength, said conducting surface being in close proximity with the outerconductor and forming a capacity therewith, the Whole tuned circuitresonating at the desired frequency in the manner of a parallelresonance circuit, and a metallic plate conductively connected to saidouter conductor and cooperating with said capacity section for enablingadjustment of the capacity of the capacity section of said innerconductor with respect to the outer conductor.

10. An oscillatory circuit comprising a concentric line resonator havingan inner and an outer conductor, said inner conductor having twosections of diiferent diameters and of approximately the same lengthconductively coupled together at only one of their adjacent ends, thelarger diameter section being folded back over the smaller diametersection, and a metallic bellows at the conductively coupled end of saidlarger diameter section, said bellows being directly connected to andeffectively increasing the length of said larger diameter section.

11. A tuned circuit comprising inner and outer conductors, said innerconductor having two sections, an inductive and a capacitive section,the whole tuned circuit resonating at the desired frequency in themanner of a parallel resonance circuit, said capacitive sectionextending over substantially the entire length of said inductivesection, said inner conductor being short and having only such length asis necessary to provide an input impedance to match the impedance of anassociated input circuit.

12. A folded, concentric resonant line comprising an outer cylindricalconductor and a central conductor, a mechanical and electricalconnection between said conductors at one of their adjacent ends, and anintermediate cylindrical conductor constructed and arranged to bemechanically and electrically coupled to the other end of said centralconductor, said intermediate conductor being so constructed and arrangedas to surround substantially the entire length of said central conductorand extending substantially parallel to said central conductor.

13. A resonant line comprising an outer conductor and a concentriccentral conductor electrically coupled together at one end, anintermediate concentric conductor electrically coupled to one of saidfirst two conductors at its other end, said intermediate conductor beingso constructed and arranged as to surround the greater part of thelength of said central conductor, and means for magnetically coupling autilization circuit to a conductor of said line at a place of greatestcurrent flow in said conductor.

14. An oscillatory circuit comprising a coaxial line resonator having aninner and an outer conductor, means for coupling said two conductorstogether at one end to constitute a tuned circuit, said inner conductorof said tuned circuit having two sections of different diametersconductively coupled together end to end, the larger diameter sectionbeing folded back over the smaller diameter section, and a coaxialconductor located between the two diameter sections of inner conductorand coupled to said outer conductor at said one end for increasing thecapacity between said inner and outer conductors.

15. A tuning device comprising a pair of condoctors disposed one withinthe other, the inner conductor of said pair having two sections ofdifferent diameters and of substantially equal length, the largerdiameter section being secured to the smaller diameter section andfolded back over said smaller diameter section.

16. A tuning device comprising a pair of conductors disposed one withinthe other, the inner conductor of said pair having two sections ofdifierent diameters and of substantially equal length, the largerdiameter section being secured to the smaller diameter section andfolded back over said smaller diameter section, and a hollow conductorlocated between said two sections of inner conductor and coupled at oneend to said outer conductor of said pair.

17. A resonant line comprising an outer conductor and a concentriccentral conductor electrically coupled together at one end, and anintermediate concentric conductor electrically coupled to one of saidfirst two conductors at its other end, said intermediate conductor beingso constructed and arranged as to surround substantially the entirelength of said central conductor, and means for coupling a utilizationcirtially the major portion of the length of the inner conductor of saidpair.

19. A concentric resonant line comprising an outer cylindrical conductorand a central conductor, a mechanical and electrical connection betweensaid conductors at one of their adjacent ends, and an intermediatecylindrical conductor constructed and arranged to be mechanically andelectrically coupled to the other end of said central conductor, saidintermediate conductor being so constructed and arranged as to surroundsubstantially the entire length of said central conductor and extendingsubstantially parallel to said central conductor, and frequencyadjusting means connected to the other end of said outer conductor.

JAMES W. CONKLIN.

